The present invention relates generally to gas sensing systems and vehicle emission detection systems. More particularly, the present invention relates to a method and system for detecting and measuring various species of hydrocarbons in a gas, such as a vehicle exhaust. The measurements detected by the present invention can be used in vehicle emission testing and other emission sensing systems and processes.
Current methods of determining whether a vehicle is compliant with emission standards include open path and closed path emissions measurement systems. In a closed path system, an emission sensor is directly connected to the vehicle, such as by insertion into a tailpipe. An open path vehicular emissions measurement system collects data by means other than a direct connection to the tailpipe, such as a remote sensor that analyzes the individual emission components as the vehicle drives by the sensor. Open path vehicle emission systems are often preferable to closed path systems because they can be used in numerous locations and do not require the vehicle to stop for testing.
It is known that vehicle exhaust contains many types of hydrocarbon compounds. Alkane compounds have only single carbon-carbon bonds. Alkene compounds have a carbon-carbon double bond, and alkyne compounds have a carbon-carbon triple bond. Aromatic compounds contain a six carbon ring with three carbon-carbon double bonds. The double bonds tend to shift within the ring, making the aromatic carbon ring somewhat resistant to destruction.
It is known that some exhaust hydrocarbon compounds also contain oxygen. Carbonyl compounds, which contain a carbon-oxygen double bond, are combustion products not found in the original fuel. If the carbon-oxygen double bond is located on the end carbon of the hydrocarbon chain molecule, the compound is an aldehyde. If the carbon-oxygen double bond is located in the middle of the carbon chain, the compound is classified as a ketone. The spectral properties of aldehydes and ketones are very similar.
Compounds containing oxygen, such as ethers and alcohols, are added to fuel because the oxygen increases the combustion efficiency and thereby reduces emissions. Ether compounds contain an oxygen atom bound to two carbon atoms. Methyl tertiary butyl ether (MTBE) is the most common ether additive. Alcohol compounds contain a single carbon-oxygen bond, and the oxygen atom is also bound to a hydrogen atom. Ethanol is the most common alcohol additive. Methanol is also blended with gasoline and used as an alternative fuel.
Current open path analysis systems determine the total hydrocarbon concentration by measuring the infrared absorption in the single carbon-hydrogen bond-stretching region. Typically, propane gas is used for calibration. However, some important exhaust species such as benzene and acetylene do not have any single carbon-hydrogen bonds. These species do not absorb in the same infrared region and thus are not measured by the current art open path emissions sensors. In addition, other aromatic and alkene species have only a few alkane groups and absorb less infrared energy than the propane standard. Consequently the current remote sensing technology generally underestimates the total hydrocarbon concentration by about fifty percent. Another problem with the current technology is water vapor interference, which especially occurs at 30 to 50 degrees Fahrenheit. Still another issue with the current art is that there is no means for correcting a gas emissions measurement for changes in ambient temperature and pressure through the sample path. Accordingly, it is desirable to provide an improved hydrocarbon measurement system as herein disclosed.
In accordance with a preferred embodiment of the present invention, a method of measuring hydrocarbon content in a gas includes the steps of measuring one or more individual hydrocarbon concentrations in a gas sample, determining a total concentration based on the measured individual concentrations, identifying whether water interference is present in the gas sample, and determining whether water concentrations exceeds the interference level. Preferably, the first and second determining steps described above includes grouping the individual hydrocarbon concentrations into one or more classes, summing the individual hydrocarbon concentrations within each of the classes to yield one or more class concentrations; and adding the class concentrations to yield the total concentration.
If the third determining step determines that the amount of water in any state of matter exceeds the interference level, the method preferably includes the additional step of relating the total concentration to water contamination. Also, if the third determining step determines that the amount of water interference exceeds a certain level, the method may include the additional step of reporting a water interference condition.
Optionally and preferably, the measuring step includes the technique of using an open path emission sensor to detect the intensity of a plurality of infrared spectra in the gas sample. The measuring step also includes measurements by one or more of the following sampling methodologies: non-dispersive infrared detection; dispersive infrared detection; non-dispersive ultraviolet detection; dispersive ultraviolet detection, and others. These sampling methodologies can employ differential optical absorption spectroscopy, and/or gas filter correlation methods of detection to determine a concentration of a gas of interest.
As an option, the method may include additional steps of scaling the total concentration to account for multiple counting of individual hydrocarbon species, adjusting at least one of the individual hydrocarbon concentrations to account for one or more ambient conditions, and/or adjusting the total hydrocarbon concentration to account for one or more ambient conditions.
The method includes a system for measuring the hydrocarbon content in a gas using either an open path or closed path emissions sensor capable of detecting a plurality of individual hydrocarbons in a gas sample, a processor, and a computer-readable carrier such as a memory medium. The computer-readable carrier contains program instructions that instruct the processor to perform the steps of receiving data corresponding to a plurality of individual hydrocarbon concentrations in a sample, determining a total concentration based on the plurality of individual concentrations, measuring water interference levels in the gas sample, and determining whether the water concentrations correspond to an interference condition. Optionally, the system also includes a transmitter that is capable of transmitting data corresponding to the total concentration.
Optionally and preferably, the computer program instructs the processor to perform the first determining step to group the individual hydrocarbon concentrations into a plurality of classes, to sum the individual hydrocarbon concentrations within each of the plurality of classes to yield a plurality of class concentrations, and to add the class concentrations to yield the total concentration. The total concentration is applied to a combustion equation that corrects for any dilution of a sample due to being exhausted into open air. Also optionally, when concentrations of water correspond to an interference condition, the program further instructs the processor to relate the total concentration to water contamination.
As an additional option, the system may include a sensor that receives data of ambient conditions. In accordance with this embodiment, the program instructions optionally and preferably instruct the processor to adjust the total concentration in response to the data of ambient conditions. Further, the emissions sensor and the processor may be linked by a communications link that allows data corresponding to a plurality of individual hydrocarbon concentrations to be transmitted by the emissions sensor to the processor via the communications link.